Protective Immunity to HIV:

What Animal Models of HIV Infection

Can and Cannot Teach Us

Models of Protective Immunity to HIVXVI International AIDS Conference

August 13 – 18, 2006Toronto, Canada

Ruth Ruprecht, M.D., Ph.D.Dana-Farber Cancer Institute

Harvard Medical School, Boston

What Are the Correlates of Protection?

What Are the Correlates of Protection?

ΛImmune

Vaccine-induced Protection Could be Due to…

• acquired immunity

Vaccine-induced Protection Could be Due to…

• acquired immunity

• innate immunity

Vaccine-induced Protection Could be Due to…

• acquired immunity

• innate immunity

• both acquired and innate immunity

Vaccine-induced Protection Could be Due to…

• acquired immunity

• innate immunity

• both acquired and innate immunity

• viral interference (in the case of live attenuated SIV or SHIV)

Passive Immunization = Tool to Assess

Correlates of Protection

Because rhesus monkeys are outbred and twin gestations are very rare, adoptive transfer of immune T cells is not feasible. Only the humoral arm of the immune system can be tested by passive immunization.

Passive Immunization: Cross-clade Protection

SHIV 1157ip: Summary• built from the SHIV-vpu+ backbone

• env was derived from an HIV clade C strain, HIV1157i, a primary R5 strain isolated from a six-month old Zambian infant who was HIV positive at birth

• replicates in rhesus macaque PBMC

• uses only CCR5 as coreceptor

• was adapted to rhesus monkeys by rapid animal-to-animal passage in five animals

• results in high peak viral RNA levels and persistent infection in infant and adult macaques

• was titrated orally in neonatal rhesus monkeys

• shows signs of pathogenicity in rhesus macaques: persistent viremia, depletion of memory T cells, loss of absolute numbers of CD4+ T cells, AIDS, thrombocytopenia

TM

gp120 gp41

SIVmac239

HXBc2

HIV1157i env

HIV1157i TM

LTRgag

pol

vifvpx

vpr vpu

1157i env

nef

LTR

tatrev

SHIV-1157i

oralSHIV-1157ip

mAbs

1hr

control (n=4)

treatment with 4x mAbs i.v. (n=4)

1 2 4

1 2 4d8

0

0

oralSHIV-1157ip

100 weeks

100 weeks

mAbs

Experimental Design

log

pla

sma

vira

l R

NA

lo

ad

(co

pie

s/m

l)

1

3

5

7

0 30 60 90

Weeks after inoculation

RPj-9

RTj-9

RWk-9

RZk-9

RPk-9

RSj-9

RTk-9

RVj-9

Week 40

Passive Immunization: 1h Post-Exposure Prophylaxis

The Correlates of Immune Protection

• The human anti-HIV mAbs used in the quadruple combination completely protected monkeys against mucosal virus challenge.

• These nmAbs, which recognize conserved epitopes and are active across clades, were the sole immune protective mechanism.

IgG1b12 : anti - CD4 binding site(b12)

2G12 : complex epitope on gp120 dependent on correct N-linked glycosylation

2F5 : linear gp41 epitope, ELDKWA

4E10 : linear gp41 epitope, NWFDIT

Epitope Specificity

Correlates of Immune Protection

IgG1b12 : anti - CD4 binding site(b12)

2G12 : complex epitope on gp120 dependent on correct N-linked glycosylation

2F5 : linear gp41 epitope, ELDKWA

4E10 : linear gp41 epitope, NWFDIT

Causes of Immune Protection

IgG1b12 : anti - CD4 binding site(b12)

2G12 : complex epitope on gp120 dependent on correct N-linked glycosylation

2F5 : linear gp41 epitope, ELDKWA

4E10 : linear gp41 epitope, NWFDIT

Protective Epitopes

IgG1b12 : anti - CD4 binding site(b12)

2G12 : complex epitope on gp120 dependent on correct N-linked glycosylation

2F5 : linear gp41 epitope, ELDKWA

4E10 : linear gp41 epitope, NWFDIT

Passive immunization with nmAbs is a tool

to identify protective epitopes

AIDS Vaccine Development

and Challenge Viruses in

Primates – Getting Real

In order to be as predictive as

possible, vaccine efficacy studies in

non-human primate models

should reflect the biology of HIV-1

transmission among humans as

closely as possible.

HIV-1 Transmission Among Humans

• 90% of all HIV-1 transmissions occur via mucosal exposure; this includes sexual transmission and mother-to-child transmission

• mucosal HIV-1 transmission involves R5 viruses almost exclusively, even if the source person harbors predominantly X4R5 and X4 strains

• recently transmitted strains of HIV-1 may be more sensitive to neutralization and encode shorter Env molecules than quasispecies that predominate in the source person (shown for clade C)1

1 Derdeyn et al., Science 2004; 303:2019-22; Li et al., J Virol 2006; 80:5211-8

Vaccine Challenge Studies in Primates:

Getting Real…

• mucosal route (intrarectal, intravaginal, oral) • R5-tropic virus• repeated low-dose vs. standard high-dose

challenges• neutralization-sensitive virus• virus lacking overwhelming, acute

pathogenicity• heterologous virus (with regard to vaccine)

Vaccine Challenge Studies in Primates:

Getting Real…

• mucosal route (intrarectal, intravaginal, oral) • R5-tropic virus• repeated low-dose vs. standard high-dose

challenges• neutralization-sensitive virus• virus lacking overwhelming, acute

pathogenicity• heterologous virus (with regard to vaccine)

Mucosal Challenge in Primates: Virus Dose Matters…

• The standard single high-dose virus challenge, designed to yield > 95% chance of infecting unvaccinated controls, does not reflect the HIV-1 inocula during sexual transmission

• High-dose challenge may overrun host defenses and set the bar for achieving protection too high

Repeated Low-dose Challenge: Lowering the Hurdle

• Example: using an R5 SHIV, Kim et al.1 performed weekly intravaginal challenges at low doses (10 TCID) in monkeys. Group 1 received the vaginal microbicide CAP 15 min prior to each virus inoculation; controls remained untreated. Whereas all controls became systemically infected after 3 to 4 weekly exposures, 3 out of 4 monkeys given CAP microbi-cide remained uninfected after 12 exposures (p = 0.015).

• CAP microbicide efficacy was 66% when tested against a single high-dose virus challenge; when tested against repeated low-dose challenges, efficacy was 92%.

1 J Med Primatol 2006; 35: 210-6 CAP = cellulose acetate phthalate.

Vaccine Challenge Studies in Primates:

Getting Real…

• mucosal route (intrarectal, intravaginal, oral) • R5-tropic virus• repeated low-dose vs. standard high-dose

challenges• neutralization-sensitive virus• virus lacking overwhelming, acute

pathogenicity• heterologous virus (with regard to vaccine)

Neutralization Sensitivity: Avoid “Stealth” Envelopes

• Late-stage viruses that have undergone multiple rounds of neutralizing antibody (nAb) selection followed by repeated escape develop more compact, hard-to-neutralize envelopes.

• Using viruses with such impenetrable envelopes in primates may set the bar for achieving vaccine protection unrealistically high.

• Neutralization resistance is an issue for SIVmac239 and primary SIVmac251 grown in rhesus monkey PBMC.

• Essentially, nAb-based vaccines cannot be evaluated for efficacy with these challenge strains.

Vaccine Challenge Studies in Primates:

Getting Real…

• mucosal route (intrarectal, intravaginal, oral) • R5-tropic virus• repeated low-dose vs. standard high-dose

challenges• neutralization-sensitive virus• virus lacking overwhelming, acute

pathogenicity• heterologous virus (with regard to vaccine)

The chance of a human AIDS vaccine

recipient to be exposed to an HIV-1

strain that exactly matches his/her

vaccine approaches zero…

Given the many HIV-1 quasispecies and their increasing divergence with time, human AIDS vaccine recipients will not encounter viruses exactly matched to their vaccine.

Vaccine efficacy testing in primates should reflect this reality. Exactly matching vaccine and challenge virus may overestimate the potential of new vaccines and raise unjustified expectations.

Success with homologous virus challenges may also stimulate the development of vaccine strategies that yield highly protective but only narrowly focused immune responses that fail to protect against divergent viruses.

Heterologous Virus Challenge: Reflecting Viral Complexity in Real Life

SHIV Challenges: Closer to the Real Thing

• SHIV chimeras allow efficacy testing of HIV-1 Env-based vaccines.

• SHIV chimeras allow testing of neutralizing antibodies isolated from HIV-1-infected individuals.

• As such, vaccine development could be significantly accelerated because primate-tested reagents can be directly used in clinical trials.

• In contrast, SIV challenges only allow the evaluation of active or passive immunization concepts but not the actual vaccines or antibodies intended for human use.

Summary

• AIDS vaccine efficacy studies in primate models should focus on mucosal challenge with R5 strains

• SHIVs have the added advantage of directly testing anti-HIV-1 Env responses

• SHIVs allow development of passive immunization with human anti-HIV-1 Env nmAbs in primates

• Challenge viruses should encode neutralization- sensitive, primary envelopes

Summary, continued

• To reflect HIV heterogeneity, vaccine and challenge virus should not exactly match. Ideally, primate vaccine efficacy studies should employ fully hetero-logous virus, rather than one differing in env only.

• Replacing single high-dose viral challenges with

repeated low-dose mucosal exposures has shown promise.

• Ultimately, efficacy data generated in primate models

need to be compared directly to phase III clinical vaccine trials for validation.

Acknowledgements

DFCI / Harvard Medical SchoolRuijiang SongRobert RasmussenAgnès Chenine

Mila Ayash-RashkovskyLauren GoinsRicky GrissonPei-lin LiChantelle McCannSaied MirshahidiHelena OngClaudia RuprechtEla Shai-KobilerTao WangJames WhitneyW. XuL.-Y. Yeh

Beth-Israel Deaconess Medical CenterLisa CavaciniMarshall Posner

Institute of Applied Microbiology, Vienna, AustriaHermann Katinger Gabriela Stiegler

Harvard School of Public HealthJanet Andersen

Memorial Sloan-Kettering Cancer CenterTing Chao Chou

Yerkes National Primate Research Center

Harold McClure† James Else Elizabeth Strobert

Centers for Disease ControlW. Evan Secor

University of NebraskaCharles Wood H. Zhang

Univ. Teaching Hospital, Lusaka, ZambiaGanapati Bhat Chipepo Kankasa

University of WashingtonShiu-lok Hu Patricia Polacino

NIAIDHIV-RAD P01: Nancy Miller